In the production of a rubber composition in which a sealed mixer is used and a raw rubber and carbon are mixed, various methods of mixing exist with an object of ensuring the stability of the rubber physical properties (see for example Japanese Unexamined Patent Application Publication No. 2011-102038A). The method of mixing described in Japanese Unexamined Patent Application Publication No. 2011-102038A is directed at a carbon introduction step in which carbon is introduced into a raw rubber, and an uniform dispersion step in which the introduced carbon is uniformly dispersed in the raw rubber. In the carbon introduction step, the integrated amount of power needed to drive a mixing rotor in rotation is detected, and once the detected integrated amount reaches a preset target amount, the process moves to the next step. In the uniform dispersion step, once the integrated amount of power needed to drive the mixing rotor in rotation reaches a preset target amount, the process moves to the next step, and in addition, a target mixing time, which is when the integrated amount of power needed to drive the mixing rotor in rotation should reach a preset target value, is determined, and the rotational speed of the mixing rotor is adjusted to reduce the deviation between the target mixing time and the estimated mixing time of this step.
In the carbon introduction step described above, when the integrated amount of power needed to drive the mixing rotor in rotation reaches the preset target amount, the ultimate temperature of the mixed rubber mixture may not be constant, which causes variations. These variations mean that variations are present in the process up until this point, and because variations in temperature are present at each progressing stage of the mixing, the rubber physical properties of the rubber composition are affected. In addition, the process of mixing the rubber mixture containing a reinforcing agent (carbon) is irreversible, and what is performed in the introduction step and the uniform dispersion step is different in nature. Thus, a problem exists in which when the variation in ultimate temperature of the rubber mixture at the conclusion of the carbon introduction step is great, the variation in the rubber physical properties of the manufactured rubber composition becomes great. There is room for enhancement involving this matter.